Question: Which of these numbers is prime? ${21,\ 57,\ 61,\ 64,\ 85}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 21 are 1, 3, 7, and 21. The factors of 57 are 1, 3, 19, and 57. The factors of 61 are 1 and 61. The factors of 64 are 1, 2, 4, 8, 16, 32, and 64. The factors of 85 are 1, 5, 17, and 85. Thus, 61 is a prime number.